The special nature of the C—C bond can lead to various polymorphic forms of carbon, such as graphite, glassy-carbon, fullerenes (such as buckyballs), carbon nanotubes and diamond. In addition to the intrinsic material properties of carbon, functionalized films can be produced through chemical modification using a wide range of chemistries. Because of this flexibility and utility, fabrication of both macroporous and microporous carbon films, with their concomitant increase in surface area, continues to receive significant research interest. Some of the specific applications for porous carbon materials include fuel cells, electrochemical double layer capacitors, high surface area catalytic supports, water purification, and gas separation.
In particular, electrodes made from carbon exhibit many useful properties including wide potential windows, low background capacitance, resistance to fouling, and catalytic activity for many analytes compared to solid metal electrodes. The nanostructuring and/or microstructuring of electrodes can lead to high-surface-area, catalytically active interfaces. Such modifications can be achieved by tuning particle size and shape, making the catalytic surface in porous networks, or dispersing catalytic particles into porous templates.
The performance of microporous carbon in the applications described above, among others, will often be limited by a tradeoff between surface area and mass transport. That is, the total (potentially reactive) surface area of a porous body of constant mass increases as the pore size decreases. Hemispherical diffusion, which is favorable for many applications involving interfacial reactions, also tends to control the mass transport when the pores are relatively small. As the pores continue to decrease in diameter, however, the penetration depth of reactant liquids may become severely limited, and due to overlapping diffusion layers, linear diffusion profiles may supplant hemispherical diffusion profiles.
Hence to optimize the performance of microporous carbon structures, it is desirable to provide not only high surface area, but also uniform and controllable dimensions.
U.S. Pat. No. 8,349,547, which issued on Jan. 8, 2013 to D. B. Burckel et al. under the title, “Lithographically Defined Microporous Carbon Structures,” and which is assigned to the assignee hereof, describes one technique for forming microporous carbon structures that is capable of providing uniform and controllable dimensions. The entirety of the abovesaid U.S. Pat. No. 8,349,547 is hereby incorporated herein by reference.
U.S. Pat. No. 8,349,547 describes a method of depositing a carbon-containing photoresist onto a substrate, lithographically defining a microporous structure in the deposited photoresist, developing the lithographically defined photoresist, and pyrolyzing the developed photoresist to provide a microporous carbon scaffold. The microporous carbon scaffold can be functionalized with, e.g., metal nanoparticles or a conducting polymer. In implementations, interferometric lithography is used to define the structures in the photoresist. The structures undergo significant shrinkage during pyrolysis, but they maintain their pattern morphology and adhesion to the substrate.
For specific applications in electrocatalysis, for example, the porous carbon material can be decorated by electrochemically depositing metal nanoparticles of, e.g., gold, palladium, or silver, or by electrochemical deposition of conductive polymers. Other methods of decorating the microporous carbon scaffolds include evaporative deposition of thin films.
The technology of composite materials offers many opportunities to make more robust structures, to incorporate new functionality into known structures, and to make new structures that could not otherwise exist. In the realm of micro-fabrication, the ability to make composite materials that are structured in the range from the microscale to the nanoscale offers particularly great opportunities for advancing this technology.
The decorated carbon scaffolds described in U.S. Pat. No. 8,349,547 provide one example of microscale or nanoscale composite materials. These materials offer great promise, even though the foreign materials added to the carbon scaffolds are generally confined to interfacial areas such as external surfaces of the scaffold and the walls and interiors of voids in the scaffold. Still greater opportunities to enhance the mechanical, electrical, optical, and electrochemical properties of the carbon structure may be enjoyed if the nanoparticles can be more intimately mixed with the pyrolytic carbon.